South Korean broadcaster YTN cited officials in Seoul as saying a launch could come on Monday.
Speculation that Pyongyang would fire its Taepodong-2 intercontinental ballistic missile at the weekend came to nothing, and forecasts of overcast skies over North Korea and possible showers on Tuesday and Wednesday could delay it.
Japan will take "severe action" if North Korea goes ahead with reported plans to launch a missile, Japan’s prime minister, Junichiro Koizumi, said on Monday.
"I still hope North Korea will not do so, but if they don't listen to us, if they fire a missile, Japan will have to take severe action in discussion with the United States," he said at a news conference.
He did not specify what the action would be. Japan has said it would seek an immediate meeting of the UN Security Council if Pyongyang test-fired a missile.
Japan's chief cabinet secretary, Shinzo Abe, said a missile test would be a threat to regional security, while US officials said Washington had sent a warning to Pyongyang through North Korean diplomats at the United Nations, but had had no response.
Australia, one of the few Western counties with diplomatic links to North Korea, summoned Pyongyang's ambassador in Canberra to express concerns.
"We are working closely with our allies and friends to try and persuade North Korea not to proceed with a test"
Alexander Downer, Australian foreign minister
"Such action would be highly provocative and would further isolate the DPRK [Democratic People's Republic of Korea]," the foreign minister, Alexander Downer, said in a statement.
"We are working closely with our allies and friends to try and persuade North Korea not to proceed with a test. We are also discussing contingencies should a test firing occur."
Pyongyang shocked the world in 1998 when it fired a missile, part of which flew over Japan and landed in the Pacific Ocean.
US officials have watched with alarm as satellite photos showing launch preparations at the Musudan-ri facility in North Hamgyong province in North Korea's northeast.
Officials in Washington, speaking on condition of anonymity, said on Sunday they believed the fuelling process was complete.
But the US ambassador to Japan, Thomas Schieffer, told reporters in Tokyo that no "definitive judgment" had been made on that point, a US embassy spokesman said.
Asked how Washington would respond to a launch, Schieffer was quoted as saying he thought sanctions would have to be considered.
"It's a very dangerous situation and it would be one that we would react to very negatively," he said.
In Seoul, across the heavily fortified border dividing the two Koreas, the daily Dong-A Ilbo quoted a Seoul government official as saying the launch could be imminent.
"We think North Korea has poured liquid fuel into the missile propellant built in the missile launching pad. It is at the finishing stage before launching," the official said.
A test is expected to involve a Taepodong-2 missile with an estimated range of 3,500-4,300 km (2,175-2,670 miles). At that range, parts of Alaska in the United States would be within reach.
North Korea lacks an operational missile that can hit the continental United States, the California-based Center for Nonproliferation Studies said in a recent report.
Reports of test preparations have come at a time of stalemate in six-country talks on unwinding Pyongyang's nuclear programmes.